A. Field of the Invention
This invention relates generally to the combination of a heat pump and a water heater and, more specifically, to the construction of a condenser assembly of the heat pump water heater and its being inserted into the tank through an existing opening in the top of the water tank.
B. Description of the Prior Art
Heat pump water heaters (HPWH) are an energy-efficient way to heat water with electricity, typically providing the same amount of hot water at one-half to one-third the energy used in electric resistance water heaters. A HPWH works by transferring heat, not be creating heat. Through a reverse application of the standard vapor compression refrigeration cycle, a heat pump water heater uses an electrically driven compressor to remove heat energy from a low-temperature heat source (ambient room air) and move it to a higher-temperature heat sink, the water stored in the hot-water tank. The energy supplied to heat the water is primarily electrical energy needed to operate the compressor. The energy supplied to heat the water comes from both the heat transferred from the ambient air and the energy used to operate the compressor in the system. Because less energy is needed to move heat than to create heat, the effective efficiency of the heat pump water heater system, defined as the ratio of hot water energy output to energy input to the water heater, is greater than 100%. The effective efficiency is called the Coefficient of Performance (COP).
A typical residential HPWH operates by extracting heat from a moderate-temperature source (such as room air), and moving it to a higher-temperature heat sink, the residence hot-water supply. This heated water is then stored in a hot-water storage tank for later use. The physics and operation of the HPWH is identical to the vapor compression refrigeration/heat pump cycle used for space conditioning heat pumps, air conditioners, and refrigerators. FIG. 2 shows the components used in a vapor compression refrigeration/heat pump cycle: compressor, condenser, evaporator, and expansion device. The flow of refrigerant between components in this closed cycle is also illustrated.
In the compressor, refrigerant vapor is compressed, thereby raising its temperature and pressure. This vapor then moves to the condenser. In the condenser, heat flows from the hot refrigerant to water surrounding the condenser. As heat leaves the refrigerant, the refrigerant condenses to a high-pressure, liquid state. The heat removed from the refrigerant as it changes to a liquid is transferred to the water.
The high pressure, liquid refrigerant leaves the condenser at a temperature slightly above the temperature of the water surrounding the condenser. The liquid passes to an expansion device, where it is rapidly depressurized, and some of the liquid refrigerant flashes back into vapor. The vaporization of a portion of the refrigerant causes the remaining refrigerant to cool rapidly, and the refrigerant leaves the expansion device as a low-temperature mixture of fluid and vapor. This cold mixture then enters the evaporator, where it absorbs heat from air blown over the evaporator coils. The liquid portion of the refrigerant evaporates, and the vapor then moves back to the low-pressure side of the compressor at a temperature slightly below the temperature of the heat source.
This continuing cycle results in movement of heat from the ambient air to the higher-temperature residential hot-water supply. In residential HPWHs, the heat source is typically air from inside the residence, although with proper duct design, the air could come from inside the residence, from outdoors, or can be set manually to come from either depending on climate conditions.
Electrical energy is required to operate both the compressor in the HPWH and a fan that continually blows air across the evaporator coils when the unit is operating. Depending on the system design, a water pump may also be needed to circulate water between the condenser and the storage tank. The compressor, however, is the major electrical load in an HPWH. Most of the energy consumed by the compressor is used to compress and subsequently heat the refrigerant vapor, with only a small fraction of energy lost as heat from the shell of the compressor. Since the total energy to the hot water comes from the energy transferred from the heat source, as well as virtually all the energy that is used by the compressor, the net amount of heat energy transferred to the hot water is considerably higher than the net input of electrical energy by the compressor. In residential HPWHs, the heat energy supplied to the water is typically between two and three times the amount of electrical energy required to operate the HPWH.
By contrast, electrical energy in a standard electric water heater is converted directly to heat in an electrically resistive element. Since the conversion efficiency from electrical energy to heat energy is 100% and the element is completely immersed in the water, the amount of heat energy supplied to the water in a standard electric water heater is equal to the electrical energy supplied to the elements. By providing more hot water per unit of electricity consumed, the HPWH saves energy and money.
Residential HPWH units are wired with electrical resistance backup for heating water during period when the HPWH will not operate satisfactorily. Backup electric resistance heat may prove necessary if the heat pump unit fails, or if the temperature of the heat source is too low for the HPWH to operate effectively. Some designs also allow the use of backup resistance heat if the hot-water load is significantly above the heat pump capacity.
There are basically two types of HPWHs currently available on the market. One is the desuperheater, which is connected to a heat pump system that is used for house cooling and heating. The desuperheater takes part of the heat from the compressor discharge gas and use it for domestic water heating. The problem with a desuperheater HPWH is that the house load might not match the water heating load. In other words, when hot water is needed, the house might not need cooling or heating, is and this results in inefficient use of the heat pump system.
Another type of HPWH is a dedicated stand alone unit. It pumps water from the water or storage tank, heats it in the HPWH using a heat pump and then circulates it back to the storage tank. While an advantage of the stand-alone is that storage tanks ox HPWH units can be replaced separately as they wear out, this type of HPWH is bulky and requires a water pump to pump water from the tank to and from the condenser. The cost for such a HPWH tends to be high. An HPWH produced by Crispaire Corp. of Norcross, Ga. (Model R106K3) is mounted on the water tank.
There is a third type of HPWH, which is not on the market yet, but has been designed and developed. In this new design, the condenser coils are wrapped around over half of the exterior of the tank wall with the balance of the refrigeration system (including controls, expansion device, compressor, fan and evaporator assembly) being mounted on top of the tank. Thus, the water is heated by heating the tank walls with the obvious disadvantage being that the condenser is not in direct contact with the water so as to have the most efficient heat transfer occur. The system is designed to be a single package, including the modified tank. This type of HPWH requires special manufacturing to wrap the copper coil around the tank wall. Also, contact resistance between wall and the coil must be minimized to insure proper system operation. Again, in case the tank must be replaced, the condenser coil will have to be cut, which involves taking refrigerant out of the heat pump first. Then, a new tank, with the coil wrapped around its wall, will have to be connected to the compressor-evaporator assembly, and then evacuation and refrigerant charging. Full replacement of the entire system will likely be the best option, and first or replacement costs could be high. Enviromaster International (EMI), with support from the Department of Energy's ENERGY STAR Program through Oak Ridge National Laboratory, is developing this type of HPWH aimed at the large electric water heater replacement market.